Remember you roof protects all the contents of your home. One should not take the cheapest price because you may pay less now but you will pay more in the future if the work and materials are not installed properly.
All roofs are different and you should ask as many questions as you can think of!
Yes but no more than 60%. If a contractor asks for more that 60% that contractor may have a money problem.
No, never go with the lowest. The contractor with the lowest bid usually missed something and will ask for more money during the job or the contractor is using substandard materials.
You should get at least three bids/proposals. Three will give you a fair idea of the cost and also a chance to interact with three contractors.
Ponding water is when a significant amount of water is present in an area for more than 48 hours.